I copied it in style.css, the color is OK, but I can still see the pictures of the slider through it when scrolling. The transpârency is still there.

Another problem is the position of the slider. It’s different in Chrome vs Edge or Firefox.

There is a whitespace between the slider and the navbar in Chrome which is not present in Firefox or other browsers.
I tried modifying the positioning of the navbar through the Slider Revolution settings, but all it does is move the slider (with the positioning delta in Chrome) which results still in the same differences in positioning.